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DELAWARE MOSQUITO CONTROL SECTION 

SPRAY THRESHOLD CRITERIA 

General Comments

What is Involved in Making Spray Decisions -- Determination of when and where 

mosquito control insecticide spraying might be needed, whether for larvicides or 

adulticides, is based upon extensive and intensive surveillance-and-monitoring efforts for 

immature or adult mosquito populations using several types of indicators for larval or 

adult mosquitoes, as well as sometimes the presence in the environment of mosquitotransmitted viruses of special concern to human health (e.g. eastern equine encephalitis, 

West Nile virus). No one factor is always decisive for when or where to spray, and often 

a combination of quantitative or objective data must be evaluated to make “spray/nospray” decisions in quasi-subjective manner (including also factoring in current or 

predicted future conditions for weather or tides that affect windows-of-opportunity for 

when spraying might best be done). 

What Does It Mean to Exceed Threshold Values -- A long history in Delaware (>45 

years) of our “real world” experiences in developing and using the threshold criteria 

below indicates that when a criterion or combination of criteria are exceeded and we do 

not initiate some control actions by larviciding or adulticiding as appropriate, then 

mosquito infestations of a level unacceptable to the public’s quality-of-life, comfort and 

well-being, and often detrimental to local economies too, will either soon ensue or 

continue unabated. Additionally, if these infestations are not satisfactorily prevented or 

controlled, there can then often be corollary substantive concerns about potential public 

health problems in the form of mosquito-borne diseases. Therefore, the numeric values 

for the thresholds provided below are always subject to further refinement or 

modifications by the Mosquito Control Section as circumstances or new information 

warrants, in that these criteria have evolved over time and will probably continue to do 

so. 

What Mosquitoes Need Control -- By no means do all mosquitoes in Delaware need 

control, since of the approximate 57 mosquito species found in the state, only about 19 

species are pestiferous or problematic to humans or domestic animals. Factors in 

determining how pestiferous a mosquito species might become include how abundant or 

numerous can the adult populations of these species become, and what are their flight 

ranges, host preferences, biting aggressiveness, or potential for transmitting diseases. 

Contending with one or more of these 19 pestiferous species results in mosquito control 

spraying in Delaware that routinely starts about mid-March with larviciding for 

woodland-pool-breeding species, then progresses to salt marsh larviciding starting as 

early as April in some years, with adulticiding of saltmarsh mosquitoes sometimes also 

needed as early as mid-April, and then continuing with various types of spraying around 

the state throughout the remainder of the control season up until as late as early 

November, including efforts to suppress container-breeding species in urban or domestic 

settings, and the control of mosquito production in myriad freshwater habitats. 
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Who Determines the Threshold Criteria -- By State statute, the Mosquito Control 

Section has the responsibility and authority for determining and using these threshold 

criteria for making decisions about when and where mosquito control spraying is needed

for any-and-all lands in Delaware, whether the lands be federal, state, county, municipal 

or private properties. This is not necessarily one-in-the-same as determining when and 

where spraying will actually be allowed to be done (e.g. on federal National Wildlife 

Refuges, the U.S. Fish and Wildlife Service exercises final authority over whether any 

spraying will be allowed), or will even be acceptable for any given landowner. 

Nonetheless, in order to effectively treat public nuisances or prevent disease, the 

Mosquito Control Section has the statutory authority to enter upon all lands under the 

State’s jurisdiction including private lands, and take whatever control actions are needed, 

including spraying without landowner permission or concurrence, doing such while 

avoiding any unnecessary damage; but for any source reduction projects involving habitat 

modifications, we try not to exercise such authority without cooperation or consent of 

landowners. 

Threshold Criteria for Application of Larvicides

Wetland or “Pothole” Larval Dipper Counts (applicable to saltmarsh or freshwater 

wetlands pestiferous species) – for larval dipper counts (using basically a coffee-cup-size 

container on a stick designed to take shallow-water samples), where larvae can range 

from 1st to 4th instars and also include pupae, our finding any larvae in >25% of all 

sampled sites that then average >5 larvae/dip (for all dips, including “zeros”) can warrant 

larviciding throughout the wetlands where the sampling runs were performed. Thus, 

these threshold criteria incorporate minimum triggers for both the spatial extent or 

frequency of any breeding observed (>25% of sampled sites), and for the intensity or 

amount of any breeding observed (average >5 larvae/dip) – i.e. for any larviciding to 

occur in a sampled marsh or wetlands, breeding should be observed in >25% of all 

the breeding sites sampled, AND be occurring at an average intensity of >5 

larvae/dip (incorporating all dips into this average). Additionally, for saltmarsh 

mosquito breeding leading to larviciding efforts, an additional factor is added whereby 

>25% of the marsh surface in “tussocky” saltmarshes, or >25% of pothole habitats in 

firmer saltmarshes, should be estimated to be inundated or holding water, as a measure of 

the extent of potential breeding habitats actually holding water. [An estimate of the % 

pupae present in saltmarsh or freshwater breeding habitats also gives us an indication as 

to whether it might be too late to attempt effective larvicide treatments.] Experience has 

shown us that if we don’t take timely larviciding actions before a mosquito brood of such 

problematic scope or intensity emerges, then it will be a matter of only a few to several 

days before adulticiding will then become necessary in either nearby or even quite distant 

locations depending upon the types of mosquito species involved, along with weather 

factors such air temperature or wind speed and direction. 

Larval Sampling Stations -- Larval sampling stations are described in our larval 

sampling plans or given via larval inspection instructions from Section managers to field 

personnel, such that timely and good spatial representation of potential breeding sites is 
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achieved, with the sampling sites selected having environmental characteristics known or 

thought to favorable for mosquito-breeding. By recent rainfall or tidal flooding patterns, 

we usually have a pretty good idea of the best timing to undertake our larval surveillance 

efforts to yield the best-return-for-effort. Refinements for the need to larvicide any 

geographic area or location where observed breeding exceeds threshold criteria can be 

made based upon the types of mosquito species found and their typical flight ranges, as 

well as a breeding site’s proximity to human population concentrations, sometimes 

resulting in our not taking larval control actions even though threshold criteria are 

exceeded. For example, long-distance-flying saltmarsh mosquitoes (often roaming 3-5 

miles up to 10-15 miles from their natal marshes) will typically warrant larviciding 

anywhere the breeding threshold criteria are exceeded; whereas some relatively shortdistance-flying freshwater species (flying from only a few hundred yards up to 1-2 miles 

from their natal habitats) might not, especially in areas remote from human populations. 

Larval Identifications -- The levels to which larval identifications need to be taken for 

operational purposes can vary depending upon the environmental setting and the 

operational utility of making such taxonomic identifications. For example, in salt 

marshes all 5 saltmarsh mosquito species encountered in Delaware are known to be 

problematic for humans, so for salt marsh surveillance any-and-all larvae can be of 

concern, and just counting larvae without regard to specific species suffices for most 

operations, which is especially true for the 3 saltmarsh Ochlerotatus (Aedes) species that 

dominate open salt marsh mosquito production, and to a lesser extent for the single Culex

and single Anopheles species found in saltmarshes. The saltmarsh Culex and Anopheles

species are more commonly associated with impounded or more permanent-water 

breeding habitats in our coastal wetlands, and also don’t fly as far as the 3 Ochlerotatus 

(Aedes) species, so unless populated areas are within 1-2 miles from where saltmarsh 

Culex or Anopheles larvae are detected, in most cases it wouldn’t be necessary to 

larvicide impounded or more permanent water salt marsh habitats (that also lacked 

significant Ochlerotatus (Aedes) breeding). Fortunately, it is relatively easy to 

distinguish in the field among Ochlerotatus (Aedes), Culex and Anopheles larvae at the 

genus level when doing dipper counts. In most freshwater environments, particularly in 

wet woodland pools, meadow swales or roadside ditches, finding and identifying in the 

field the larvae of Ochlerotatus, Aedes or Psorophora genera can indicate a problem 

regardless of species; and when populated areas are found within 1-2 miles of such 

freshwater breeding habitats, so can finding and identifying in the field larvae of Culex or 

Anopheles genera regardless of species. Finding larvae of Ochlerotatus, Aedes, 

Psorophora, Culex or Anopheles genera regardless of species in or near populated areas, 

whether the larvae are found in standing water, ephemeral water, or container-breeding 

habitats (including for the latter any man-made structures holding water for more than 4 

consecutive days, or in natural treeholes), can all be problematic. Overall for operational 

purposes in almost all types of breeding habitats, identifying larvae in the field just to the 

genus level quite nicely suffices. Finally, it should be noted that finding larvae of 

Coquilletidia perturbans can also be indicative of a problem situation, but because of 

their larval requirements for burying into sediments and attaching to rooted aquatic 

vegetation, these larvae are rarely observed in the field without purposely sampling for 
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them, and such larval habitat also does not lend itself to larviciding, so adulticiding 

becomes the primary form of control for this species. 

“Container” Occurrences (applicable to larvae of pestiferous species found in natural or 

man-made containers or other water-holding structures) – any larvae of pestiferous

species (genera) observed in or sampled from container-like habitats might warrant 

control, especially if close to human habitations and standing water will remain for more 

than 5 consecutive days. Primary species of concern will be those found in genera Culex, 

Ochlerotatus or Aedes. If the “containers” lend themselves to dipper sampling, then 

larval counts averaging >2 larvae/dip might warrant control actions. If larval control 

cannot be achieved by source reduction (e.g. dumping or eliminating standing water), 

then larviciding might be needed, almost always done via ground- or hand-applications 

(exceptions involving aerial applications might occur for salvage yards, tire dumps, or 

sewage lagoons). 

Threshold Criteria for Application of Adulticides

Landing Rate Counts (applicable to adult biting mosquitoes landing on a field inspector 

during a one-minute period in or nearby human habitations or within flight ranges of 

human population concentrations, with the counts taken anytime between early morning 

through early evening) – in populated areas (such as cities, towns or suburbs, or in 

exurban subdivisions or larger strip-developments), landing rate count averages for biting 

mosquitoes of >2-3 mosquitoes/minute (translating into 60-90 or more mosquito bites 

in a half-hour in somebody’s backyard) can trigger the need for adulticiding; whereas in 

relatively unpopulated areas (often rural locations with low population densities, and 

commonly in or near wet woodlands or marshy staging areas), landing rate averages for 

biting mosquitoes of >5-10 mosquitoes/minute (becoming 150-300 or more mosquito 

bites in only a half-hour) might be needed to warrant adulticiding. [In some locations at 

certain times, landing rate counts of 50-100 mosquitoes/minute have been recorded!] 

The results obtained with landing rate counts can be almost immediately diagnostic of a 

need to adulticide. 

 

How to Perform Landing Rate Counts -- The protocol for conducting landing rate 

counts consists of counting all mosquitoes observed landing on all readily visible parts of 

an inspector’s body in one minute intervals, with the inspector standing still and making 

very little movement, along with not using any type of repellent. At any particular site, 

specific standing locations for conducting the counts are selected favoring shaded spots 

near vegetation having little or no wind. When it is obvious that a landed mosquito is 

either biting or about to bite an inspector, the inspector is allowed to undertake subtle 

movements during the count to terminate or prevent the bite (which makes for very 

interesting movements when dozens of mosquitoes might be simultaneously trying to 

bite!); and care is also taken to the extent possible not to count any single mosquito more 

than once during any one-minute count. 

When to Take Landing Rate Counts -- Because of normal working hours for State 

employees, most landing rate counts are conducted during the daytime, which in allowing 
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for commuting time then leads to most counts being conducted in the field between about 

8:30 am and 4:00 pm. This can work well for saltmarsh and wet woodland Ochlerotatus 

(Aedes) spp., for freshwater Psorophora spp. and Aedes vexans, and for the containerbreeding Aedes albopictus, but often tends to underestimate the abundance on-wing 

that’ll be found in the early evening, throughout the night, or early in the morning for 

Culex and Anopheles spp., and to a lesser extent for Coquilletidia perturbans. Whenever 

possible and practicable to do, particularly where pertinent for dealing with potential 

“hot” situations, landing rate counts are also conducted near or at dusk or in the early 

morning. But it can be safely assumed that any mosquito landing upon an inspector is 

one of the 19 problematic species, and hence any-and-all mosquitoes observed alighting 

during a landing rate count are included in the tally. 

New Jersey-style Adult Light Trap Counts (collected via unbaited NJ adult light traps 

at a few dozen “permanent” locations around the state, typically set within populated 

areas, or sometimes set in locations indicative of potential problems for nearby populated 

areas) – having nightly NJ adult light trap counts containing >25 females/trap of 

pestiferous species, checked the following day after a night’s trapping period, or which 

average >25 females/trap of pestiferous species for multi-night collections, are indicative 

of adult mosquito populations on-wing in the general vicinity that would have been 

intolerable to most people the night(s) before. Because of the lag time involved with 

collecting and analyzing adult light trap collections, these counts often are not as 

immediately diagnostic of any need to adulticide as are landing rate counts; but within 

about 48-72 hours of problems arising from excessive numbers of adult pestiferous

mosquitoes on-wing, the light trap counts can serve as indicators of the need to 

adulticide, or can help document how effective an earlier control treatment might have 

been. However, even with such a delay in immediate operational utility, the light trap 

counts are still valuable for allowing us to examine many of the types of pestiferous

mosquitoes that might be around, at least for the trap-susceptible species, allowing for 

year-to-year comparisons at any given location and month-to-month comparisons within 

any given year. 

CDC-style Portable Adult Light Trap Counts (collected via CO2-baited portable CDC 

adult light traps at temporary collection stations around the state, often set and tended in 

location types similar to where NJ adult light traps are deployed, and often set in response 

to other indicators of mosquito problems, particularly to determine or to verify “hot 

spots”) -- nightly CDC adult light trap counts collected the following day containing >50 

females/trap of pestiferous species are indicative of adult mosquito populations on-wing 

in the general vicinity that would have been intolerable to most people the night before. 

[The CO2-baited nature and other inherent design features of the CDC-style traps cause 

them to collect more mosquitoes per night than NJ-style adult light traps, but the NJ-style 

traps are more durable and less costly to operate than the CDC-style traps, and the State 

of Delaware also has a much longer historic database using NJ-style traps at wellestablished, fixed locations.] Because of the urgency with which these portable CDCstyle traps are often set and tended to and then analyzed, they can be diagnostic of the 

need to adulticide within about 24-36 hours of their setting. 
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Public Complaints About Biting Mosquitoes – these are called into or otherwise 

communicated to either our upstate or downstate Mosquito Control Section operational 

headquarters (in Glasgow for New Castle County and northwestern Kent County, and in 

Milford for the remainder of Kent County and Sussex County) by citizens or elected 

officials, and usually involve people wanting to report or complain about unacceptable or 

intolerable numbers of biting mosquitoes on-wing, followed by their then requesting 

some adulticiding (although sometimes the reports involve concerns about observations 

of standing or stagnant water bodies that might produce mosquitoes, which they either 

want us to inspect and treat with larvicides or to somehow eliminate). Receiving 

complaints from the public is an invaluable way to help us focus and make best use of 

our limited control resources – depending upon weather conditions and other 

environmental factors during any given year, on a statewide basis the Mosquito Control 

Section typically receives from about 1500-3000 public complaints about too many biting 

mosquitoes, conveyed to us from early April into early November. 

Reactions to Public Complaints -- Depending upon patterns for the geographic 

locations, densities and intensities of public complaints received, the Mosquito Control 

Section might then undertake some adulticide spraying. When practicable to do within 

limits of our staff availability and working resources, before making any final decision to 

spray just based upon public requests, we often try to integrate these calls for treatment 

with other available mosquito-problem indicators, such as landing rate counts or adult 

light trap counts. Over the period of many years of our doing this, we have also come to 

know many individuals whose requests for some adulticiding relief to suppress mosquito 

populations are unfailingly accurate and representative of truly intolerable quality-of-life 

conditions caused by too many biting mosquitoes, and we tend to pay extra attention to 

many of these regular “trusty” callers. We also take quite serious those requests coming 

from city or town officials for our adulticiding services, for those municipalities that have 

endorsed our annual Spray Policy’s requirements and protocols. And to repeat here, 

whenever possible and practicable to do, our responding to public complaints by actually 

spraying is also first coupled with other indicators of adult mosquito abundances, 

including landing rate counts or adult light trap counts pertinent to the areas where 

adulticiding requests have arisen. 

Indicators of Mosquito-Borne Diseases

The Mosquito Control Section conducts surveillance-and-monitoring for mosquito-borne 

diseases of note to humans that in Delaware primarily concerns two arboviruses -- eastern 

equine encephalitis (EEE) and West Nile virus (WNV). 

Sentinel Chickens -- The Section operates a statewide network of a few dozen “sentinel 

chicken” monitoring stations each year, whereby from about late May into late October 

or early November about one-half of our sentinel birds are tested every week for the 

presence of antibodies indicative that they have might contracted EEE or WNV, yielding 

for us a good signal of the viruses’ presence and transmission within the environment. 

The sentinel chicken flock locations are selected to give us good geographic coverage (or 

at least as much as we can afford) for general areas either known or suspect to have good 
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virus potential. Each sentinel chicken station (or “flock”) consists of a total of 4 birds 

that are humanely housed and cared for in 2 wire mesh cages per station, with 2 birds 

each week from each station having small blood samples drawn for virus testing, thereby 

causing any individual chicken to be sampled but once every two weeks. The bleeding 

does not kill or harm the birds, and any chickens that might contract EEE or WNV also 

do not die from these viruses. 

Sick or Dead Wild Birds -- The Mosquito Control Section also operates from April 

through October a statewide network for collecting and reporting sick or dead wild birds 

suspect to have WNV, relying primarily upon public reports of suspect wild birds, 

collection of good candidate birds by the Section, and testing for the presence of WNV 

by the state’s Division of Public Health Laboratory. The Section has a statewide 

geographic strategy for accepting and testing wild birds that helps to ensure good spatial 

coverage and timely monitoring. 

Mosquito Collections -- Occasionally on an ad hoc basis, the Mosquito Control Section 

will also analyze mosquito collections for the presence of WNV or EEE, sometimes with 

an interest in the specific species of mosquitoes carrying the viruses, or sometimes with 

only a more general interest in documenting the presence of viruses at the genus or guild 

levels. Samples for such analyses are usually collected by portable adult light traps. This 

might be done in conjunction with or as follow-up to other indications of virus presence, 

particularly in known or suspect virus “hot spots.” 

Horse/Human Cases -- The Mosquito Control Section also receives timely reports from 

the State Veterinarian for the finding of EEE or WNV in unvaccinated horses; and we 

receive from the state’s Division of Public Health timely reports of any EEE or WNV 

human cases. 

What Does Presence of Virus Mean? -- Indications of the presence of EEE or WNV by 

any of the methods above is then factored both spatially and temporally into the types and 

extent of control actions that we take. In part we use such indications to try to increase 

the public’s awareness to take some personal protection measures against being bitten, 

and for them to also practice good water sanitation around their homes or businesses to 

reduce mosquito production. We also use such virus indications to increase to the extent 

possible and practicable our surveillance-and-monitoring actions for assessing mosquito 

populations, and to increase our mosquito control treatment efforts accordingly, since one 

of the more important factors in preventing mosquito-borne diseases is to reduce even 

further populations of bridge-vector species, in order to decrease the probability even 

further that people will get bit by disease-carrying mosquitoes (i.e. good nuisance control 

= good disease prevention or control). This increase in our control responses relative to 

findings of virus presence is typically achieved by our then reducing the spray threshold 

criteria associated with indicators of mosquito abundance, in that we then undertake 

spray actions at lower larval dipper counts, lower landing rate counts, lower adult light 

trap counts, lesser numbers of public complaints, etc. than what have been indicated 

above for when there are no indications of disease. By how much we actually lower 

these threshold criteria in response to virus presence is fairly subjective, based upon our 
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semi-quantitative estimations of virus presence; but since human health is involved here, 

we of course tend to-err-on-the-side-of-caution (within limits of our available treatment 

resources and applicable environmental considerations). 

Last revision – June, 2008